Topsoil: What is it and who cares?

Topsoil means many things to many people, but to everyone it represents the best part of the soil from a plant-growth perspective. Many activities alter the soil profile including surface mining, agriculture, and urban development. Of these, mining is subject to state and national regulations for protection of soil and the USDA has a series of programs to protect topsoil from erosion. The extensive use of mass grading to remove topsoil from entire subdivisions during construction will likely create pressure for additional standards and regulations governing topsoil protection and replacement, as will national efforts to restore brownfields. Topsoil is the subject of mine reclamation regulations and is viewed as something to be protected and preserved, but also something that regulators will allow, in certain situations, to be removed or buried and replaced by a topsoil substitute. When there is a need for a suitable growth medium to support vegetation at a site that has lost its native topsoil due to mining or other earth moving activities, a wide range of materials can be used as topsoil, including subsoil or selected overburden materials. The Surface Mining Control and Reclamation Act (SMCRA) was the first federal statute to specifically define operations involving the handling, storage, and substitution of topsoil. Within SMCRA, "topsoil" is not specifically defined, but the A horizon is identified in the prime farmland subsection, and by implication it is topsoil. SMCRA also specifically allows for the use of topsoil substitutes when the pre-mining A ⁺ E horizons are less than 15 cm thick. Blasted sedimentary overburden materials are routinely converted into successful topsoil substitutes in the Appalachian coal mining region, but compaction commonly limits their productivity, and post-mining pH must be carefully matched to intended post-mining vegetation. A wide range of organic and mineral wastes and residual products can be beneficially used for either in-situ soil reconstruction or on-site remediation. Similarly, many run-of-mine mineral wastes can be successfully combined with organic composts to produce commercially viable manufactured topsoils. This paper will review the authors' experience with "topsoil", both in a scientific and practical, applied sense. Of necessity, it will focus on surface mine issues, while raising other issues and discussing some case studies.